1. Field of the Invention
In attempting to develop new technologies for economical storage of energy, the sodium/sulfur battery offers many advantages. The sodium/sulfur battery is rechargeable, has large storage capacity, and employs relatively inexpensive materials of low atomic weight, so that storage capacity per unit weight is greater than heavier elements, such as lead.
While the properties of sodium and sulfur as electrodes, in combination with a beta-alumina solid electrolyte have many instrinsic advantages, they also provide many new and unique problems. The sodium/sulfur battery must operate at elevated temperatures, of the order of 300.degree. C. or above. At these temperatures, both sodium and sulfur can be highly reactive and corrosive with a wide variety of materials. Furthermore, electricity must be introduced into and removed from the sulfur electrode at these temperatures. However, sulfur is nonconductive, so that means must be provided for the introduction and removal of electrons.
It has further been found, that during discharge and recharge, a film of sulfur may build up at the electrolyte surface, so that passage of electricity to the electrode is inhibited by the high resistance offered by the sulfur adjacent the electrolyte.
Faced with these unique problems associated with the nonconductive sulfur electrode, a solid electrolyte, and high temperatures of operation, various new approaches have been required to provide for efficiently operating batteries.
Even where advances have been made in the structure of prototype Na/S cells, assembly procedures useful in producing small prototype Na/S cells are not necessarily feasible for the production of full-size cells. One such example, is the known methods for preparation of the graduated resistance sulfur electrode. The advent of thermocompression bonding (TCB) has resulted in the sulfur compartment being attached to the alpha-alumina insulator prior to the insertion of the sulfur electrode. In small prototype cells this compartment is sealed after the sulfur electrode is formed. Proper operation of the sulfur electrode demands that the carbon fibers produce continuous contact between the beta-alumina tube and the container wall throughout the length of the solid electrolyte. Thus any process which requires that a pre-cast electrode plug be slid into place over a length of 15 inches is fraught with the potential for problems, such as regions of poor contact. Moreover, the precasting of the electrode plug in and of itself has problems and costs associated with molding and machining of the plugs to specific dimensions.
2. Description of the Prior Art
U.S. Pat. No. 4,070,527 describes the use of graduated resistive material in a sulfur electrode, specifically mats of carbon fibers having differential resistivity. U.S. Pat. No. 4,053,689 describes the use of a carbon fiber mat for contacting a molybdenum or chromium coated surface of an aluminum conductor to provide electrical conductance through the sulfur electrode. U.S. Pat. No. 4,048,390 is cited as of interest.